1. Field of the Invention
The present invention is generally directed to a method to identify the constellation type of a modulated signal.
2. Background
Communication systems employ a variety of signal modulation. These include frequency independent types such as pulse amplitude modulated (PAM) signals, quadrature amplitude modulation (QAM) and phase-shift keying (PSK).
The number of discrete waveforms may be denoted as M points in the state space constellation. In PAM, the ergodic (i.e., time-dependent) signal waveform sm(t) may be represented by a periodic cosine function that includes a series of discrete amplitudes ranging from m=1, 2, . . . , M. For pulsed signals sent over discrete time intervals, the time t may be replaced by nT where T is the sampling period and n is a positive integer. PAM is similar to vestigial side band (VSB) used in television. The M-PAM signals may be plotted in single dimensioned signal space R (also written as R) at M discrete points along the real axis. The energy for the waveforms may be a proportional to the square of the amplitudes.
FIG. 1 shows a 4-PAM signal space diagram 10 with an axis 12 for example M=4. The data points are presented for a binary bit pair having four possible positions at points 16a and 16b on the left side of the imaginary axis and 16c and 16d on the right side in this example. Phase modulated signals, such as PSK and QAM, may be multi-dimensional belonging to signal space RN, where N represents the dimension superscript.
For N=2, there exist real and imaginary components for representing amplitude and phase. A distance d from the origin of a point on the two-dimensional signal grid in R2 can be expressed in complex form as d=di+jdq where di is the in-phase or real component, dq is the quadrature or imaginary component and j={square root over (xe2x88x921)}.
In PSK, the signal waveforms sm(t) have equal energy and can be plotted as being equal distant from the origin. FIG. 2 shows an 8-PSK signal space diagram 18 for M=8 with data points 20a and 20b on the real axis 12, 20c and 20d on the imaginary axis 14, and 20e, 20f, 20g, and 20h at intermediate positions.
QAM may include a series of discrete amplitudes in addition to a phase component to distinguish a set of four points by the quadrant occupied. FIG. 3 shows a 16-QAM signal space diagram 22 for M=16 with the axes 12 and 14 dividing the space into four quadrants 24a, 24b, 24c and 24d. The first quadrant 24a includes four points 26a, 26b, 26c and 26d. The second quadrant 24b includes four points 26e, 26f, 26g and 26h. The third quadrant 24c includes four points 26i, 26j, 26k and 26l. The fourth quadrant 24d includes four points 26m, 26n, 26o and 26p. 
For M=4, data values to be represented may range from two-digit binary numbers 002, 012, 102, and 112. For M=8, data values to be represented may range from three-digit binary numbers 0002, 0012, 0102, . . . , 1112. For M=16, data values to be represented may range from four-digit binary numbers 00002, 00012, 00102, . . . , 11112. The incoming signal to be interpreted as these data values may be received as voltages or digital numbers, with each discrete data value corresponding to a particular range of voltages.
Several communication transmission media are widely in use today, such as satellite, microwave, terrestrial, and cable systems. These transmit data at a variety of data rates. In order to convert the signals from received voltages into data, their amplitude and phase must be resolved. This task may be complicated by electronic noise from a variety of sources. A receiver configured to resolve only a select signal constellation would be unable to resolve an alternate modulation constellation. Smaller integrated chips could allow a wider variety of modulation systems to be received, if these are identified by modulation type. Accordingly, there exists a need for accurate and efficient detection of the modulation constellation type of a signal to enable multi-mode multi-standard operation of communication receivers.
A method for determining the signal constellation of a received signal establishes a moment based on each waveform, squares the moment, fourth powers the moment, divides the fourth power by the square to obtain a ratio, and compares the ratio to a threshold. If the ratio is less than threshold, the signal constellation corresponds to a first type, and if greater than the threshold, the signal constellation corresponds to a second type. The method can be generalized to a magnitude mean in place of a second moment.